Merge branch 'difference_lemmas' into 'master'

Some lemmas about `difference` and `delete`

See merge request !5
4 jobs for master in 6 minutes and 34 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #3221
fp-timing
stdpp-coq8.5.3

00:02:23

passed #3220
fp-timing
stdpp-coq8.6

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passed #3219
fp-timing
stdpp-coq8.6.1

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passed #3218
fp-timing
stdpp-coq8.7

00:04:58